1. Field of the Invention
The present invention relates to a transceiver for performing direct conversion, time division, full duplex, spread spectrum communication according to digital wide band modulation and demodulation associated with a differential binary phase shift keying technique.
2. Description of the Related Art
Referring to FIG. 1, an antenna 110 is shown which both receives signals from the air and transmits modulated and amplified signals into the air. A time division duplexer 112, operatively coupled to the antenna 110, switches between transmission mode and receiving mode in a given time interval. The transceiver is set to receiving mode when a switch of the duplexer 112 is switched to a first terminal S1. The transceiver is set to transmission mode when the same switch is switched to a second terminal S2. Switching of the switch of the duplexer is controlled by a control signal C1 provided by a controller (not shown) which controls the transceiver overall. A low noise amplifier LNA 114, operatively coupled to the terminal S1 of the duplexer 112, amplifies a received signal from the time division duplexer 112 by a given level. A first band pass filter BPF 116 receives the amplified received signal and passes only the portion of the signal which is within a given frequency band. Further, a filter 124 receives a square wave signal which includes pseudo noise data (PN+data) and converts such signal to a sine wave signal which includes the PN+data by low pass filtering.
A first phase-locked loop PLL 120, operatively coupled to the filter 124, obtains a fme variable frequency by frequency synthesizing using a stabilized oscillator. The first PLL 120 oscillates a high frequency modulated by the PN+data of the sine wave signal from the filter 124. A first frequency mixer 118, operatively coupled to the first PLL 120 and the first BPF 116, produces a first intermediate frequency signal by attenuating the signal received from the first BPF 116 by the oscillation frequency from the first PLL 120. The second BPF 122, operatively coupled to the mixer 118, passes only the first intermediate frequency. A signal identical to the control signal C1 of the time division duplexer 112 is received and switched to the first switch terminal S1 in receiving mode and switched to the second switch terminal S2 in transmission mode by the first PLL.
A second phase-locked loop PLL 130 produces first and second oscillation frequencies VS 1 (first VS) and VS 2 (second VS) by a control voltage CV transmitted from outside. At this time, the second PLL 130 transmits the first oscillation frequency VS 1 in receiving mode and the second oscillation frequency VS 2 in transmission mode in conjunction with the signal identical to the control signal C1. A demodulator 126, operatively coupled to the second PLL 130 and the second BPF 122, converts the first intermediate frequency passed from the second BPF 122 to a second intermediate frequency via synthesis with the first oscillation frequency VS 1. The demodulator 126 then amplifies such signal and demodulates it to the PN+data. A second frequency mixer 132, operatively coupled to the first PLL 120 and the second PLL 130, produces a transmission signal by synthesizing the second oscillation frequency VS2 from the second PLL 130 and the oscillation frequency from the first PLL 120. A third BPF 134, operatively coupled to the mixer 132, transmits only a transmission signal within a desired band after receiving the transmission signal from the second frequency mixer 132. A power amplifier 136, operatively coupled between the third BPF 134 and terminal S2 of the duplexer 112, amplifies the transmission signal passed from the BPF 134 by a desired power factor.
The time division duplexer 112 transmits a signal received by the antenna 110 when it is switched to the first terminal S1, in receiving mode, by the control signal C1. The received signal is amplified by the LNA 114, filtered by the first BPF 116 and provided to the first frequency mixer 118. The received signal is converted to the first intermediate frequency signal by the first PLL 120, filtered by the second BPF 122 and converted to the second intermediate frequency signal by the demodulator 126 to become the modulated PN+data. The second frequency mixer 132 produces a transmission signal by synthesizing the second oscillation frequency VS2 from the second PLL 130 and the oscillation frequency from the first PLL 120. Further, the transmission signal is filtered by the third BPF 134 and amplified by the power amplifier 136. At this time, the time division duplexer 112 is switched to the second switch terminal S2 and the amplified transmission signal is transmitted through the antenna 110 into the air.
As a result of this double conversion time division method employed in conventional transceivers, time division duplex (TDD) noise is produced due to the on/off operation of the second PLL 130. Further, there exists unnecessary frequency components due to spurious and harmonic ingredients produced in the two oscillation circuits. Still further, there is considerable circuit complexity associated with the demodulator 126 due to the use of the first PLL 120 and the second PLL 130.